Study on magnetic tunnel junction

Magnetic tunnel junctions (MTJ), i.e., structures consisting of two ferroma gnetic layers (FM1 and FM2), Separated by a very thin insulator barrier (I) , have recently attracted attention for their large tunneling magnetoresist ance (TMR) which appears when the magnetization of the ferromagnets of FM1 and FM2 changes their relative orientation from parallel to antiparallel in an applied magnetic field. Using an ultrahigh vacuum magnetron sputtering system, a variety of MTJ structures have been explored. Double H-c magnetic tunnel junction, NiFe/Al2O3/Co and FeCo/Al2O3/Co, were fabricated directly using placement of successive contact mask. The tunnel barrier was prepare d by in situ plasma oxidation of thin At layers sputter deposited. For NiFe /Al2O3/Co junctions, the maximum TMR value reaches 5.0% at room temperature , the switching field can be less than 10 Oe and the relative step width is about 30 Oe. The junction resistance changes from hundreds of ohms to hund reds of kilo-ohms and TMR values decrease monotonously with the increase of applied junction voltage bias. For FeCo/Al2O3/Co junctions, TMR values exc eeding 7% were obtained at room temperature. It is surprising that an inver se TMR of 4% was observed in FeCo/Al2O3/Co. The physics governing the spin polarization of tunneling electrons remains unclear. Structures, NiFe/FeMn/ NiFe/Al2O3/NiFe, in which one of the FM layers is exchange biased with an a ntiferromagnetic FeMn layer, were also prepared by patterning using optical lithography techniques. Thus, the junctions exhibit two well-defined magne tic states in which the FM layers are either parallel or antiparallel to on e another. TMR values of 16% at room temperature were obtained. The switchi ng field is less than 10 Oe and step width is larger than 30 Oe.